1. Field of the Invention
The present invention relates generally relates to series wound motor controls and, more particularly to a solid state series motor control.
2. Description of the Related Art
Series wound motors have the armature and field winding connected in series. The operating speed of these motors is inversely proportional to the imposed load. This characteristic makes them desirable in traction drive applications, where a high starting torque is required to overcome a large inertia.
Controls for series wound motors are often the constant potential type. These controls are referred to as constant potential because the line-to-line voltage remains unchanged during motor operation. The controls apply a potential (voltage) to the motor armature at each speed point by shunting out preset segments of resistance in series with the motor. However, the motor speed is also related to field strength which is dependent on the load due to voltage drop across the series resistance). A weaker field results in faster motor speed. When high values of series resistance are used to restrict motor speed in slower speed points, a heavily loaded motion may not begin to move until the control is placed into second or third speed point. Series wound motors have a unique application on traction drives that must negotiate curves. When individual motors in same positions (opposite each other) are connected in series, they develop the same torque but allow the speeds on the inner and outer wheels to conform to the severity of the curve.
Controlling the speed and direction of series wound motor with solid state devices instead of the original contactor resistor control systems has been satisfied by reversing one section of the motor with an H-bridge topology and controlling the other section with an independent sold state switch. The motor is then controlled as a separately excited machine and uses a microprocessor with current feedback from both the armature and the field section to reconstruct the series motor characteristic. Usually the armature section is within the H-bridge and the field winding is switched independently and the system attempts to maintain the armature and field currents at the same value.
This separately excited machine configuration exposes the motor to potential damage. If the software is not functioning correctly or the current feedback is not accurate, the field current could then be much less than the armature current. With moderate differences during drive the motor can over speed and with significant differences the armature is not subject to sufficient magnetic coupling. The armature then becomes a rotating short circuit that causes significant damage to the commutator and brush gear. Since the field winding is not connected in series with the armature, rapid changes in the applied voltage to the armature can cause very high di/dt changes resulting in high voltages across the commutator segments with risk of damage to the commutator.
The true series connection always has the very inductive field winding in series with the armature and the motor itself limits the di/dt even if rapid changes in applied voltage to the motor do occur. With the motor driving, should the field current suddenly increase, then the armature voltage can rise rapidly above the supply voltage causing significantly high voltage on the commutator and exposing other devices connected to the same supply to an over voltage condition. The same conditions that caused the motor to generate result in an unexpected braking effect which can damage the equipment being driven by the motor.
The separately excited design requires at least four insulated gate bipolar transistors (IGBTs), or similar semiconductors, to form the H-bridge and one IGBT and a diode to switch the field. To deal with the potential of the regenerated voltage a second IGBT and diode are required to operate an over voltage dump system. Since the field and armature are controlled independently, the losses in the system are high causing thermal issues that need to be dealt with adding more complexity and cost. In applications where the energy source is limited, i.e. a traction battery, this inefficient system limits the operational time between battery changes. Therefore, a need exists for a solid state series motor control to control the speed and direction of a series wound motor while maintaining the series connection of the armature and the field winding.